The present invention, in some embodiments thereof, relates to material science and, more particularly, but not exclusively, to diamond-coated substrates, processes of preparing same and uses thereof.
The industrial use of diamond films for coating various substrates increases constantly due to its special characteristic properties. Diamond is a unique substance owing to its exceptional optical, mechanical, thermal and electrical properties, and particularly due to its structure which confers high hardness and good resistance to mechanical wear, damage and mechanical stresses. The diamond's high wear resistance makes it a preferred candidate for high-impact applications in the form of thin diamond coatings on hard substrates for making tools such as, for example, drill bits, surface polishing pads, cutting edges and blades, earth-boring machines and various other articles and tools.
Diamond-coated articles are commonly prepared by forming a layer of polycrystalline diamond (diamond film) to the surface of a substrate material. Polycrystalline diamond films can be formed by chemical vapor deposition (CVD) of carbon at specific temperatures and pressures. Carbon CVD is essentially based on contacting a surface of a substance susceptible to carbon vapors, namely to a carbon-containing gas. A substance susceptible to carbon vapors is a substance which is not inert to the carbon-containing gas, and thus do not retard or prevent carbon absorption into a surface thereof, which is needed for initial diamond formation and the subsequent diamond film deposition and growth. On the other hand, if a substrate is prone to absorb carbon, this advantageous trait may become disadvantageous since an accelerated carbon absorption during carbon deposition may tip the dynamic equilibrium such that carbon accumulation on the surface of the substrate is disfavored, and/or such that competing thermodynamic processes, favoring the formation of carbides and/or the more thermodynamically stable graphite, inhibit diamond formation.
Examples of carbon vapors-susceptible metallic substrates, which allow carbon absorption, include high-speed steel (HSS) or stainless steel (SS), which are essentially alloys of iron (Fe), carbon (C) and other alloying elements. Examples of carbon vapors-absorbing composite substrate materials include cemented/sintered carbide or nitride substances, such as tungsten carbide (WC) and silicon nitride (Si3N4), which have a structure of carbide or nitride grains bound together by a metallic binder (“cement”).
One example of a highly suitable composite substrate material for carbon absorption, opening the possibility to deposit a diamond film thereon, is tungsten carbide cobalt alloy (WC—Co), or tungsten carbide cemented with cobalt. In the case of substrates made of WC—Co, cobalt is present mostly between the carbide crystals as a binder, but it also covers some of the WC grains owing to the excellent wetting properties of Co and WC, hence practically, there is no WC surface free of cobalt in such a cemented carbide. Tungsten carbide (WC, often referred to colloquially as carbide) is an inorganic chemical substance containing equal parts of tungsten and carbon atoms (stoichiometric mixture). Tungsten carbide is approximately three times stiffer than steel, with a Young's modulus of approximately 550 GPa, and is much denser than steel or titanium. It is comparable with corundum (α-Al2O3) or sapphire in hardness and can only be polished and finished with abrasives of superior hardness such as cubic boron nitride and diamond amongst others, in the form of powder, wheels, and compounds. Tungsten carbide is high melting, 2,870° C. (5,200° F.), extremely hard (8.5-9.0 Mohs scale, Vickers hardness number of 2242) with low electrical resistance (about 2×10−7 Ohm·m), comparable with that of some metals such as vanadium 2×10−7 Ohm·m). Molten cobalt can maintain contact with the solid surface of WC, which can be sintered to form the composite material WC—Co. Investigation of the phase diagram of the WC—Co system shows that WC and Co form a pseudo binary eutectic. The phase diagram also shows that there are so-called η-carbides with composition (W,Co)6C that can be formed and the fact that these phases are brittle is the reason why control of the carbon content in WC—Co hard metals is important.
Diamond coatings are typically performed by chemical vapor deposition (CVD) which is typically carried out at elevated temperatures, e.g., 600-800° C., in the atmosphere of methane and hydrogen as a source of atomic carbon and hydrogen. The formation of diamond films by CVD onto non-diamond substrates consist of two sequential processes: nucleation and growth. The nucleation process is usually achieved by seeding the substrate surfaces with very small diamond particles in the nanometer range. The growth or CVD process consists of exposing the surface to the activated hydrocarbon at well defined substrate temperatures and deposition time. The thickness of the diamond films is determined by the particular deposition conditions, i.e. degree of gas activation and composition at the growing surface, substrate temperature and deposition time.
Good adhesion of the deposited diamond coatings onto appropriate substrates is an important factor in all diamond-coated tool and device applications. Without strong bonding to the substrate, the coating will not endure harsh mechanic stress and will eventually delaminate.
One cause for poor adhesion of the diamond film to the substrate is thermal expansion of the composite system. Upon cooling from diamond deposition temperature to ambient temperature, the dimensions of the coated object change due to shrinkage, while the extent of shrinkage is governed by the coefficient of thermal extension (CTE). Typically the CTE's of the diamond coating and that of preferred substrates differ substantially. For example, HSS and SS substrates have CTE of approximately 13×10−6, and cemented carbide substrates have CTE of about 3×10−6, both large values compared with the CTE of about 0.3×10−6 measured for diamond. Hence, due to large difference between the CTE's of the coating and substrate, the substrate shrinkage is more extensive compared to the diamond's coating film, and therefore the diamond coating develops high level of compressive residual stress. This incompatibility in CTE and resulting residual stress buildup limit the thickness of the diamond coating and typically leads to delamination, namely detachment of the film from the substrate.
Thus, due to diamonds exceptional mechanical properties of low thermal extension coefficient, high compressive strength, hardness and wear resistance, compared to the properties of the substrate's material, existing diamond-coated tools and articles typically fail due to insufficient strength of the interface between the diamond coating and the substrate, rather than wear of the diamond coating itself. Therefore, one of the persistent problems still plaguing modern diamond coating technologies is the generally poor adhesion owing to stresses which result from thermal expansion.
Another cause for poor adhesion of the diamond coating film to the substrate stems from chemical considerations. It has been established that iron and cobalt, which are typically used as binders in cemented carbides, act as catalysts converting the diamond into graphite. This detrimental catalytic effect leads to the formation of the layer of graphitic carbon and degrades the adhesion of diamond to steel and cemented carbide substrates. As a result of the described catalytic affect, the direct deposition of diamond on steel or cemented carbide substrate results in the formation of a non-adhering layer of graphitic soot covered by poor-quality diamond. Also, high diffusion rate of carbon atoms into the iron- or cobalt-containing substrate leads to loss of carbon atoms from the interface, leaving voids behind and degrading the interface strength even further.
Thus, current diamond coating methodologies which utilize cemented carbide substrates also suffer from incompatibility between the chemistry and microstructure of the diamond coating and that of the substrate due to carbon diffusion and the formation of a non-adhering layer of graphite.
To achieve a good adhesion between the diamond film and the carbide substrate, it is essential to limit the carbon-cobalt interaction with the CVD conditions and therefore with the diamond film. In addition, in order to obtain good adhesion between the diamond film and the substrate, it is essential to relieve the thermo-mechanical stress during cooling, and limit the carbon-catalyst interaction during the CVD conditions. To date, this has been achieved by (i) depletion of the catalyst from the surface; or (ii) by applying an interlayer between the substrate and the deposited diamond film. Other attempts at overcoming the chemical incompatibility included use of alternative carbide binders.
For example, Shi, C. R. et al. [Diam. Rel. Mat., 4 (1995) 1079-1087] teach a continuous well-adhering high-quality diamond films grown on sintered tungsten using a Ni—Fe binder.
U.S. Pat. Nos. 5,523,158, 5,523,159, 5,547,121, 5,567,525 and 5,738,698 teach the use of a vanadium containing braze to increase the reliability of a braze joint formed between a diamond film and a tungsten carbide surface.
The use of interfacial layers, or interlayers, acting as diffusion barriers has been proposed to improve the adhesion of diamond coatings to some of the aforementioned materials. These layers are interposed between the substrate and the diamond coating. However, some of these interfacial layers do not form chemical bonds with diamond and therefore do not provide good adhesion between the diamond coating and the substrate. As a result, the adhesion between diamond coating and the substrate remains a problem. Moreover, some interfacial layers do not provide stress relieve for diamond coating and do not prevent the residual stress buildup due to incompatible CTEs. Thus, upon the cooling to ambient temperature the diamond coating deposited on a substrate with some interfacial layer is under high residual stress, which limits the coating thickness and may lead to its delamination, as mentioned above.
One solution to the problem of diamond deposition onto steel is utilization of an intermediate layer between the substrate and the deposited film. which consists of a nitridized chromium and results in a chromium nitride interlayer composed of a composite crystalline CrN and Cr2N film, as described, for example, in Fayer, A. et al., Appl. Phys. Lett., 67, 1995, 2299-2301; Weiser, P. S. et al., Surf. Coat. Techn., 71, 1995, 167-174; Glouzman O., et al., Diam. Rel. Mat., 6, 1997, 796-801; Gluzman, O. et al., Diam. Rel. Mat., 7, 1998, 597-602; Glouzman O., et al., Israel J. Chem., 38, 1998, 75-84; Glouzman O., et al., Diam. Rel. Mat., 8, 1999, 859-864; and Kreins, L. et al., Diam. Rel. Mat., 13(9), 2004, 1731. In these studies it was found that for the interlayer to afford formation of well adhered diamond film, the interlayer must act as a diffusion barrier, and therefore must be thick enough, about 20 microns, in order for it to fulfill that role. However, although diamond film was formed on steel utilizing a chromium nitride interlayer, the required thickness of the interlayer meant that fine structural features in the substrate, such as corners, curves and sharp edges, typically measuring in the 5-50 micron range, would become “fuzzy”, loosing definition due to the thickness of the thick interlayer.
U.S. Pat. No. 7,195,817 teaches diamond-coated steel or cemented carbide substrates, wherein adhesion of the diamond film is improved by use of a combination of interlayers disposed between the diamond coating and the substrate in order to alleviate thermal expansion stresses and inhibit the diffusion of carbon and other species between the substrate and the forming diamond film. The proposed combination of interlayers is taught to provide both mechanical stress relief based on the presence of a metallic interlayer, and inhibition of graphite formation based on the presence of a ceramic interlayer. However, this reference fails to provide working examples which were shown to provide the desired result.
Additional background art includes U.S. Pat. Nos. 8,147,927, 8,147,927, 7,879,412, 5,921,856, 5,763,879, 5,997,650, 6,454,027, 6,617,271, 5,952,102 and 5,681,783, Lux, B. and Haubner, R., Pure & Appl. Chem., (1994), 66(9), p. 1783-1788 and Han S. et al., Thin Solid Films, (2000), p. 578-584.